Autoimmune and allergic diseases afflict millions of people worldwide. Novel immunosuppressive strategies to prevent and treat these diseases are urgently needed. Following several years of studying the potent immunosuppressive effects of TCDD, we recently discovered that activation of the transcription factor, Aryl Hydrocarbon Receptor (AHR), by TCDD, in CD4+ T lymphocytes leads to the induction of a unique subpopulation of highly immunosuppressive regulatory T cells (Treg). These AHR-induced CD4+CD25+ Treg were more potent than natural CD4+CD25+Foxp3+ Treg in vitro. The induction of AHR-Treg appears to underlie the potent suppression the graft-vs host (GVH) response in TCDD-treated mice. Furthermore, treatment of NOD mice with TCDD prevented the development of diabetes, in parallel with increased frequency of CD4+CD25+ T cells in the pancreatic lymph nodes. These data support the hypothesis that the AHR signaling pathway is a novel pathway for the induction of Treg that may be amenable to manipulation for therapeutic benefit. The goal of the proposed studies is to understand the signaling pathways that are influenced by AHR activation in responding CD4+ T cells that lead to the induction of AHR-Treg, and to use the pattern of changes in gene expression to identify other AHR ligands that might also induce AHR-Treg. Specific Aim 1 will define the changes in gene expression in CD4+ T cells that are induced by TCDD-mediated AHR activation in vitro under a variety of well-defined conditions of T cell differentiation. The second specific aim will validate the functional significance of key genes that are identified in Specific Aim 1 to support the development of a custom gene expression array for identifying other AHR ligands that induce Tregs. Specific Aim 3 will identify novel ligands for the AHR using a unique 3-tiered approach to ligand screening that includes prioritization based on lack of effects on T cell proliferation. In Specific Aim 4, we will screen novel AHR ligands for their ability to induce TCDD-like changes in gene expression in vitro during CD4+ T cell differentiation and test positive chemicals in vivo for their ability to induce AHR-Treg during an acute GVH response. These studies are timely and important in the context of understanding the pathways for the development of adaptive Tregs as the AHR may represent a useful target for therapeutic intervention in the prevention and treatment of a wide variety of human diseases. At the same time, they will greatly enhance our understanding of the molecular mechanisms underlying TCDD-induced immune suppression and may lead to useful biomarker(s) of an "immunotoxicity pathway" for dioxin-like chemicals. PUBLIC HEALTH RELEVANCE: This research will characterize a newly discovered pathway for inducing regulatory T cells and will identify new ligands for the receptor that activates this pathway. Such ligands hold promise as new therapies for treatment of immune-mediated diseases such as allergic asthma, multiple sclerosis, type 1 diabetes, lupus and psoriasis.